1. Field of the Invention
The present invention relates to an error detection method for detecting an error of the control information to be transmitted via a downlink channel of radio communication and a mobile terminal, and more particularly to an error detection method for detecting an error of the control information to be transmitted via HS-SCCH of HSPDA, and a mobile terminal.
2. Description of the Related Art
HSDPA (High-Speed Downlink Packet Access) is one high-speed packet transmission technology which is positioned as a 3.5G standard, with respect to the 3G (third generation) standard using W-CDMA, standardized by “Release 5” issued by 3GPP, the 3G system standardization group, in March 2002. In HSDPA, a technology for high-speed scheduling of downlink packets according to such information as the channel quality of the terminal (e.g. portable telephone) and QoS, increasing the speed of shared channels (HS-DSCH: High-Speed Downlink Shared Channel), decreasing delays due to retransmission processing when a communication error occurs (H-ARQ: Hybrid Automatic Repeat Request), is used. In a conventional W-CDMA, a data communication speed of 384 kbps is the upper limit, and increasing the speed under the normal 3G standard of 2 Mbps is the limit, but by using HSDPA, speed can be increased to 14.4 Mbps (downlink) in theory.
An increase in communication speed is basically implemented by automatically selecting a higher speed modulation scheme and encoding scheme according to the status of the radio waves. Specifically, when the status of a radio wave is not good, modulation by low-speed QPSK (Quadrature Phase Shift Keying) with high stability and an encoding scheme with a high error correction capability (low-speed because of high overhead) are automatically selected, and when the status of a radio wave is good, modulation by a faster 16 QAM (16 Quadrature Amplitude Modulation) and an encoding scheme, with low error correction capability (high-speed), are automatically selected. For retransmission control, a hybrid ARQ (Automatic Repeat Request) system is used, so as to suppress the retransmission count when an error is detected.
The downlink in HSDPA is the HS-DSCH (High-Speed Downlink Shared Channel), which allows a plurality of users to share one channel. HS-DSCH is a transport channel, and the corresponding physical channel is comprised of the HS-PDSCH (High-Speed Physical Downlink Shared Channel) and the HS-SCCH (High-Speed Shared Control Channel). HS-PDSCH is a channel for transmitting data, and HS-SCCH is a channel for transmitting control information.
The data transmitted via HS-DSCH (information bits) is encoded (channel coding) by the transmission side (base station), and on the encoded bit string, rate matching processing is performed for mapping onto the physical channels.
FIG. 1 is a diagram depicting a configuration example of the H-ARQ processing unit, which executes the rate matching processing, of the transmission side (base station). The HSDPA performs encoding processing in the H-ARQ processing unit (rate matching processing unit) 1, the turbo encoding unit 10 turbo-encodes the HS-DSCH data (information bit N) to be input, and parallel-outputs three types of signals: system bit Nsys0 (state where information bit N is not processed), parity 1 bit Np10 (state where an information bit is turbo-encoded), and parity 2 bit Np20 (state where an information bit is turbo-encoded and interleave-processed). Among these three types of signals, first rate matching processing is performed on the parity 1 bit Np 10 and the parity 2 bit Np 20 in the first rate matching processing unit 11. Therefore from the first rate matching processing unit 11, the first rate matching-processed parity 1 bit Np1 and first rate matching-processed parity 2 bit Np 2 are output, and these bits are held in the virtual IR buffer 12 once. The system bit Nsys, which is not first rate matching-processed, is also held in the virtual IR buffer 12.
Then second rate matching processing is performed on each bit string Nsys, Np1 and Np2 by the second rate matching unit 13. The system bit Nt, sys, the parity 1 bit Nt, p1 and the parity 2 bit Nt, p2, after second rate matching processing, are converted into serial bit strings Ndata by the parallel-serial conversion unit 14, and are mapped onto the physical channels. In the first rate matching processing, rate matching is performed only for puncture, and rate matching is not performed for repetition. Puncture is a processing for extracting a bit from a bit string at a predetermined cycle, and repetition is a processing for inserting a bit into a bit string at a predetermined cycle.
If the modulation scheme is QPSK, Ndata is 960×P bits, and if 16 QAM, it is 1920×P bits (P: code multiplexer count 1, 2, . . . , 15) and a minimum of 960 bits. The choice between QPSK or 16 QAM is specified by the parameter Xms (modulation scheme information) <1 bit included in the control information of the physical channel HS-SCCH>, and the code multiplex count is calculated by a parameter of HS-SCCH, that is Xccs (channelization-code-set information) <7 bits>.
The range of the size of the virtual IR buffer 12 is 800 bits, 1600 bits, . . . , 14400 bits, 15200 bits, 16000 bits, 17600 bits, . . . , 28800 bits, and 30400 bits, and the size of the virtual IR buffer 12 is specified by a parameter from the higher layer.
The size of the information bit N is a transparent block (TrBLK) size plus a CRC bit (24 bits fixed). The TrBLK size is calculated based on the modulation scheme which is known by the parameter Xtbs (transport-block size information) <6 bits> and the parameter Xms of HS-SCCH and the code multiplex count which is known by the parameter Xcss, and can be a value in the 137 bits-2792 bits range. Therefore the size range of the information bit N is 161 bits-2816 bits, since a 24 bits fixed CRC bit is added.
The receive side (mobile terminal, such as a portable telephone), on the other hand, receives and decodes the HS-DSCH data signal Ndata from the transmission side. For this decoding, the above mentioned various parameters included in the control information of HS-SCCH must be used. In the HS-SCCH, a CRC bit (16 bits fixed) is added, and if an error is not detected in the CRC check, decoding is executed using the parameters included in HS-SCCH, but if an error is detected in the CRC check, decoding is not executed judging the parameters of HS-SCCH as unreliable.
The CRC check has a probability of an error, which is supposed to be detected, but may not be detected, and in theory this probability is ½16 if the CRC bit is 16 bits. In this case, the data must not be decoded since the parameters of the HS-SCCH are unreliable. In other words, a parameter error of HS-SCCH must be detected by a means which is different from the CRC check.